In the PON technology, it needs consistent upstream burst overhead parameters (i.e., parameters of upstream burst overhead) in the OLT and the ONU: the ONU sends upstream frames according to the determined upstream burst overhead parameters; the OLT correctly receives the upstream frames also according to the determined upstream burst overhead parameters. Based on this premise, the PON can work properly. Generally, the upstream burst overhead parameters include:
(1) State (on or off) of Forward Error Correction (FEC);
(2) Delimiter, which has a variable length and can be suitable to different optical fiber link quality;
(3) Preamble, which typically is a constant, but also may be several constants for selecting, and relates to different optical fiber link quality and performance of the optical module; and
(4) Guard band (Guard time), which is the number of bits of the guard band, and generally has a constant length.
In the existing ITU G.984.3 GPON technology standard, these upstream burst overhead parameters are determined between the OLT and the ONU according to the following method: in a register process which starts up periodically, the OLT broadcasts a part of the upstream burst overhead parameters required by the OLT, including the parameters (2), (3) and (4) in the above-mentioned upstream burst overhead parameters, to all ONUs which communicate with the OLT. As shown in FIG. 1, after the ONU acquires downstream physical layer synchronization and enters into a waiting state (i.e., O2 state), it continuously monitors a message carrying the part of the upstream burst overhead parameters broadcasted by the OLT in the downlink; once the message is received successfully, the ONU records the carried upstream burst overhead parameters, and then transfers to a serial number state (i.e., O3 state); and the ONU uses these received upstream burst overhead parameters in the subsequent working process without modification.
In addition, in the existing Gigabit Passive Optical Network (GPON) standards, state notification mechanism of the parameter (1) FEC in the upstream burst overhead parameters is independent from communication mechanism of the above-mentioned parameters (2), (3) and (4). The on/off notification of the parameter (1) FEC is indicated in real time by a fixed indication bit in a tag field assigned for each upstream burst bandwidth of each ONU in a bandwidth assignment field of the downstream frame.